The present invention relates generally to personal security systems, and more particularly to a personal security system employing a locating and tracking device.
Security personnel often place their lives at risk merely by showing up for work. Quick response to emergency situations helps alleviate the concern that some of these personnel feel when performing their jobs. Yet, at times these individuals are unable to notify the main control center that they are in trouble or where they are when trouble arises.
College campuses and certain employers also seek to monitor security personnel within their zone of control, both for safety and managerial reasons. Some of these areas are inside buildings, while others are outside.
Many organizations would benefit from the ability to continuously locate the position of individuals as they move throughout a facility. The most likely candidates of these are corrections facilities, hospitals, and nuclear power plants and storage sites. However, university and office campuses and amusement parks also become candidates for this capability as the frequency of violent crime increases.
Early tracking and direction-finding (DF'ing) was achieved using directional antennas or pseudo-Doppler array techniques. These techniques were developed for outdoor use over relatively long ranges. The reliability and accuracy of these systems falls short of required personnel tracking needs.
The increase in violent crime and the imposition of mandatory sentencing guidelines have dramatically expanded the U.S. population behind bars. The U.S. prison population climbed to 1.4 million in 1996--a 100% increase over a seven year period. Expansion of prison capacity has failed to keep pace despite $2 billion national budgets for new prison construction. Advanced technology is needed to protect corrections staff in the current over-crowded prison settings and to extend electronic monitoring of prisoners on early release and home-arrest programs.
The plague of violent crime has extended from the country's urban centers to envelop commercial business centers, government facilities, shopping malls and university campuses. Local police are overwhelmed in their attempts to protect an increasingly mobile populace. Nationwide, 911 emergency services are unable to adequately respond to requests for aid. In Los Angeles, 13.5% of 911 callers abandon their call before operators service their request for emergency aid.
Introduction of remote emergency alarm reporting and locating technologies could relieve the burden on existing law enforcement agencies and deliver more rapid response to true emergency situations. In an agreement with the Cellular Telecommunications Industry Association, the FCC mandated a five year program requiring cellular providers to phase in technology to determine the location of cellular callers to within 125 meters. Similar personnel location services are required on a micro-cellular basis.
Direction-finding (Angle of Arrival)
The earliest position location systems were based on direction-finding (DF) antenna technology. These systems calculated the angle of arrival of incoming radio signals. If two or more receive sites were equipped with DF receivers, and the relative positions of the two receive sites was known with accuracy, the location of the transmitter could be computed by calculating the intersection of the two angles of arrival. This calculation is often referred to as "triangulation."
Modern DF systems utilize a pseudo-Doppler technique to determine the angle of arrival of an incoming radio signal. A useful way to visualize the concept is to imagine a fixed monopole antenna mounted on the circumference of a rotating disk. During some portions of its travel, the monopole will be moving towards the transmit site. During other portions, the monopole will move away from the transmit site. The received carrier frequency will exceed that of the transmitter while the monopole is approaching the transmit site due to the Doppler effect. The received carrier frequency will be less than the that of the transmitter while the monopole is moving away from the transmit site--again, due to the Doppler effect. The observed output frequency from the rotating antenna will be modulated at the rotational frequency of the monopole. If the output signal from this monopole is demodulated, the result will be an audio tone equal in frequency to the rotational rate of the monopole antenna. The relative phase of this tone, however, is determined by the angle of arrival of the transmitted carrier. By performing a phase comparison between this demodulated tone and the physical rotational frequency of the monopole, the angle of bearing will result.
Large mechanical rotating structures are undesirable from a reliability standpoint. Practical implementations replace the single rotating monopole with an array of fixed dipoles which are sampled using an electronic commutator. This multi-element antenna is called an Adcock array.
FIG. 1 presents a block diagram of an Adcock array connected to an antenna commutator, a DF receiver and bearing processor and display. The receiver is a standard FM receiver tuned to the frequency of the transmitter. It separates the recovered Doppler tone from any modulation (audio/data) applied by the transmitter. The bearing processor contains a phase comparator which operates on the antenna commutator control signal and the demodulated output of the receiver.
When this process is applied to locate tracking beacons, the transmit pulse width is very narrow (50-250 ms) with a maximum duty cycle of 10%. Pulsed operation maximizes tracking beacon battery life. The challenge for DF receivers is to capture sufficient pseudo-Doppler information and perform the bearing calculation within the pulse width of the transmitter. Pseudo-Doppler principles apply at any frequency of operation. However, practical limitations on antenna construction and RF propagation confine commercial DF systems to the HF through UHF bands.
The principal limitation of pseudo-Doppler systems is susceptibility to multipath (RF carrier reflections from adjacent objects). The DF receiver will determine the angle of arrival of the largest amplitude signal reaching the antenna array. Due to multipath, the angle of arrival of the largest signal often deviates from the actual bearing of the transmitter. In high multipath environments, such as dense urban areas, DF receivers often produce erroneous results. Indoors, where direct line-of-sight paths seldom exist, pseudo-Doppler DF receivers and triangulation are useless. In addition, the Adcock arrays are physically large and should be mounted on masts for maximum performance. Such installations are generally unwelcome inside buildings.
The bearing accuracy of commercial DF systems is typically 3.degree. RMS under ideal, line-of-sight conditions. At a range of one mile, this bearing accuracy corresponds to a location uncertainty of 276 feet. This accuracy degrades dramatically under multipath conditions. To achieve a higher degree of accuracy, the DF receivers need to be spaced more closely. For twenty foot accuracy, the maximum distance between the DF receiver and the transmitter is 380 feet.
DF systems can locate individuals moving through large, open compounds if their movements are confined outdoors. Used in conjunction with an indoor location capability, DF systems can extend coverage into distant, outdoor regions of a large facility. By themselves, DF systems can not provide a single solution to personnel location in facilities having indoor areas.
Satellite-based Radiolocation
Satellite-based location systems utilize an array of communication satellites to locate earth-bound personnel location devices PLD's. Location and velocity data are computed by calculating the relative range between the individual satellites and the PLD and the Doppler shift of the received carriers. The most mature of these system, Global Positioning System (GPS), delivers accurate time, location, and velocity data to receive-only hand-held units allowing simultaneous access by an unlimited number of earth-bound units. Other systems have been recently deployed which utilize an array of low earth orbit (LEO) satellites to provide bidirectional data communications and positioning information to commercial users.
Global Positioning System
GPS was developed to deliver accurate position, velocity and clock data using a standard positioning reference to an unlimited number of simultaneous users distributed throughout the globe. In 1990, there were only thirteen orbiting GPS satellites in service limiting the system's availability to eighteen hours per day. Full service will be provided by twenty-one satellites providing twenty-four hour, three dimensional positioning around the globe. Basic accuracy is ten meters for the military service and twenty-five meters for commercial GPS receivers. Differential GPS service can improve the accuracy dramatically but differential GPS service requires communication with a second fixed terrestrial GPS station.
GPS satellites transmit signals on two L-Band frequencies (approximately 1.5 GHZ) enabling the system to adjust for variations in ionospheric propagation time delays. Users obtain three-dimensional co-ordinates based on ranging measurements from at least four GPS satellites. Velocity information is extracted from the Doppler shift of the received GPS carriers. GPS utilizes a spread spectrum coding format allowing the satellite transmissions to occupy the same spectrum while enabling the earth-bound GPS receivers to independently track the carrier phase and code.
FIG. 2 presents a block diagram of a GPS-based personnel location device. The PLD obtains its location from the GPS receiver. Upon activation, the PLD transmits this location, along with an ID code to the central monitor site.
Due to the requirement for the GPS receiver to acquire, decode, and process four independent satellite signals, GPS receivers require over one minute to determine position after power up. After acquiring this first bearing, position updates occur much more rapidly. A one minute delay is not compatible with personnel location applications where emergency response is required. Typical response times for emergency location systems are three seconds maximum. A potential solution would be to operate the GPS receiver continuously to maintain position information current. However, the power consumption (900 mW) of existing OEM GPS receiver boards is too high for continuous operation from body-worn communications battery packs.
GPS service is effectively confined to outdoor location. The link margins are so small that the RF carrier penetration loss into most building materials reduces the GPS signal level below the receiver detection threshold. For example the penetration loss produced by standard cinder block is 21 dB at 1.5 GHZ. Recall that accurate location requires four satellite signals to reach the GPS receiver simultaneously. Even if the receiver is operated outdoors, care must be taken to direct the receive antenna upward--this restriction is not practical for most body-worn PLD applications.
GPS accuracy can be seriously degraded by carrier reflections off the ground and nearby objects. The reflected signals combine with the direct path signals to create a propagation phenomenon known as "multipath." The multipath effect causes a slow fading (0.1 Hz variation in received signal amplitude envelope) and, in spread spectrum systems, corrupts the code and carrier synchronization. The following results were obtained by Van Nee:
______________________________________ Receiver Environment SNR B.sub.f Mean Error ______________________________________ Rural, Suburban, Fixed 5 dB 0.1 Hz 33 meters ______________________________________
In other words, in rural and suburban settings using a motionless GPS receiver, the amplitude of the direct path signal exceeds the multipath signal by 5 dB, and a .+-.33 meter error occurs at a 0.1 Hz rate (once every 10 seconds). In urban settings, where the streets are lined with tall buildings, the multipath delay spreads are shorter and the mean error due to multipath will be even larger.
An error of this magnitude, .+-.33 meters, exceeds the maximum tolerable location error for most personnel location applications. As stated, if differential GPS service is available, the accuracy can be improved significantly. However, combining the limited location accuracy with the restrictions of outdoor-only service and one minute location-determination delays, it becomes apparent that GPS is not a complete solution to current personnel location applications. GPS can be a valuable outdoor complement to other indoor location capabilities such as the tagging or distributed sensor approaches, which are discussed later herein.
Signpost Location Systems
If the facility to be monitored has well defined patterns of movement and many bottleneck areas which personnel must traverse during their movements, then that facility is a candidate for a signpost location system. At each of these bottlenecks, a "signpost" is installed which logs the passing of individual body-worn Personnel Location Devices (PLD's). The system's location accuracy is dictated by the number of installed signposts and by the precision with which a given signpost predicts an individual's location. For example, a signpost installed at an entry point to a small office would yield a more accurate location than a signpost installed at an entry point to a large auditorium.
One feature of all signpost systems is that they can only provide updated position information when PLD's pass by the electronic signposts. Interrogating the signposts will yield updated information only if a PLD has traversed a signpost since the last report. Signposts have unique advantages and disadvantages with regards to reliability. If one signpost fails, the location capability is terminated in that area--there is no systematic redundancy. However, the degraded location capability is confined to one zone and the accuracy of the remainder of the system is unaffected. One unattractive feature of signpost systems is their lack of an effective self-test mechanism. The only way to confirm operation is to physically walk through the facility and test location accuracy.
There are two variations on the signpost architecture shown in FIGS. 3 and 4, which are typical of all location systems. FIG. 3 presents a tagging style system, in which the PLD acts as a transmitter, which announces to the system its location. FIG. 4 presents a self-locating system in which the PLD acts as a receiver and determines its location from the signpost. The PLD then transmits this location information to the central monitor site upon alarm activation. These two approaches will be discussed in detail in the following sections.
Tagging PLD's
FIG. 3 presents a tagging system where the PLD acts as a transmitter which announces to the system its location. For reliable operation, the required PLD transmit duty cycle would be so high that battery life of a PLD using an active transmitter would be compromised. Therefore, a transponder-based PLD is utilized. Many commercially available systems based on RFID technology can be applied to the signpost location problem. For example, the Texas Instruments Registration and Identification System (TIRIS) supplies tags in a variety of form factors. One large-volume application of the TIRIS technology is automobile security. Commercial tags operate in the 150-400 KHz bands and the FCC ISM bands.
The PLD shown in FIG. 3 contains a passive transponder, which reflects a portion of the incident RF interrogating signal. The reflected signal is modulated by a serial data sequence identifying the PLD. The only power required by the PLD is that consumed by the ID code generator and a bias current for the transponder subcarrier oscillator. No RF carrier is actually generated by the transponder itself in an attempt to minimize PLD current drain. The signpost in this case is actually a CW radar. The radar consists of an interrogator, which generates the transmitted RF signal and receives the reflected carrier. The radar then subtracts a sample of the transmitted carrier to yield the ID code of the PLD passing within the radar's illumination zone. An ID decoder then formats the ID for retransmission. A communications interface encodes the signal for modulation and transmission over a wired or wireless network to the central monitor site. The range system was limited to about 15 feet.
Self-locating PLD
FIG. 4 presents a self-locating system, in which the PLD acts as a receiver and determines its location from the signpost. In this case, the signpost continuously transmits a location code that the central monitor site will interpret correctly as a particular entry way or room number. Although this location code could be transmitted on an RF carrier, current commercial implementations utilize IR and ultrasonic carriers in an attempt to confine the propagation of the location signal. These carriers however can easily be blocked by clothing. The wearer of the PLD must be careful to avoid obstructing the path between the PLD and the signpost by garments--a limitation found unacceptable to some users. Further, IR receivers are susceptible to saturation by direct or reflected sunlight so operation is typically confined to indoor areas.
The PLD in this configuration contains a receiver/decoder, which collects the location signal and, in effect, tells the PLD where it is currently located. The PLD then formats the location code packet with a PLD identifier code packet and transmits the information over a wired or wireless network to the central monitor site. This transmitted data packet informs the central monitor site of the current location the PLD.
Time of Arrival Location Systems
Tracking systems under development utilize distributed receivers, which measure the time of arrival of personal alarm transmissions. The systems addressed here are terrestrial location receivers, which can be installed throughout urban areas or building complexes. The speed of travel of radio signals in free space is known--signals require one nanosecond (ns, one billionth of a second) to travel one foot. Knowing this delay, and knowing the exact positions of an array of time-of-arrival (TOA) receivers, one can compute the location of an RF transmission based on the relative times-of-arrival of the signal reaching each receiver. The term "relative" must be emphasized because the actual time of the RF transmission is unknown--only the time at which the wavefront reached each receiver is known. Therefore, the calculation is not as simple as finding the intersection of multiple circles. A minimum of three receivers must receive the RF transmission to obtain a single location solution.
TOA receiver arrays must be synchronized to perform relative time-of-arrival measurements. For indoor or urban receivers, this requires a hardwired connection between the receiver modules to synchronize the time stamp clocks. For example, given the propagation speed of RF signals (1 ns/foot), the clocks of every TOA receiver must be synchronized to within 20 ns to achieve a location accuracy of 20 feet. Further, the receivers signal strength indicator must be sampled at an even higher rate (less than 5 ns) to prevent sampling error from contributing further to the location inaccuracy. Installation of this synchronization backbone can be extremely expensive--particularly in existing corrections facilities in which cabling must be installed in conduit and security procedures hamper access. In addition, cutting this backbone can potentially disable the complete system.
TOA location systems are particularly vulnerable to multipath. When a single direct and multiple reflected signals reach the TOA receiver, the solution is simple--the direct signal will always travel over the shortest path and will provide the best estimate of relative transmitter distance. The TOA processor must always choose the relative time-of-arrival of the first signal to reach it. However, in most urban and indoor settings, there is no direct path. Because of the complex signal propagation paths, the first signal to arrive does not necessarily represent the Euclidean distance to the transmitter.
A further difficulty arises in TOA systems when attempting to process multiple alarms over a short period. The problem becomes apparent when examining the typical delay spread of an RF signal in an indoor setting. Delay spread is the range of RF carrier propagation delays between two physical points in space caused by the many propagation paths that exist between two points. FIG. 5 presents the results of a propagation delay profile over an indoor obstructed path (no line of sight exists).
The indoor delay profile demonstrates that, although the first time-of-arrival occurs at 50 ns, multipath causes delayed replicas of the direct signal to be received for the next 200 ns. These delayed signals are still within 20 dB of the direct signal. If a distant alarm transmission were to take place within this 200 ns window, its measured time-of-arrival would be corrupted. The alarm transmitters would need to operate using a carrier sense-multiple access (CSMA) protocol to eliminate this problem. CSMA requires the personal location device (PLD) to monitor the channel for activity before transmitting. Although feasible, it now requires the PLD to include a receiver which is constantly powered, which limits battery life and increases package size.
The TOA approach imposes another limitation for personnel tracking applications. Flexible location systems are designed to locate existing personal communications devices (cell phones, walkie talkies) to minimize system cost. These communications devices are characterized by long periods of continuous transmission. During continuous transmission periods, the TOA system is unable to measure a relative difference in carrier arrival times and the transmitter's position cannot be updated.
The present invention is therefore directed to the problem of developing a method and apparatus for locating a person within a particular room inside a building or, if outside, within a small enough radius that enables a rapid identification of where that person is located.